1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft attitude control systems.
2. Description of the Related Art
The diagram 20 of FIG. 1 illustrates a conventional spacecraft 22 that orbits in an orbital plane 23 about the earth 24. Although the spacecraft""s orbital plane may be coplanar with the earth""s equatorial plane 25, it is shown, for generality, as having an inclination 26.
Spacecraft generally carry a payload which facilitates performance of the spacecraft""s intended service. An exemplary service provides communications for a service area (e.g., the area 28 on the earth 24) and, accordingly, the spacecraft""s body 30 carries a corresponding payload 32 which includes antennas 33. The body also carries solar panels 34 which provide electrical current to operate the payload and an attitude control system which maintains a spacecraft service attitude that is necessary to carry out the service. The attitude control system generally includes an attitude determination system which generates an attitude estimate in response to attitude measurements from spacecraft attitude sensors.
As spacecraft have evolved, their service demands have generally increased which has caused their payloads to become more complex so that many spacecraft now carry multiple payloads. Despite great care in design and fabrication, small relative attitude errors generally exist between these multiple payloads. More significantly, relative attitude errors are generated by differential thermal heating as the spacecraft rotates relative to the sun and passes in and out of the earth""s shadow. This differential heating induces thermal deformations and consequent relative attitude errors that often exceed the spacecraft""s design budget. Conventional spacecraft systems that determine a single attitude cannot remove these significant relative errors.
The present invention is directed to attitude control methods and systems for multiple-payload spacecraft. The invention recognizes that payload attitudes must be determined to reduce relative attitude errors between payloads. The invention thus provides an integrated attitude determination architecture that combines attitude measurements from all payloads to determine a master attitude estimate for a master payload and relative slave attitude estimates for the remaining slave payloads. These estimates are then used to control the attitudes of spacecraft elements that correct the absolute and relative attitude errors.
It has been found that systems of the invention significantly enhance the accuracy of the attitude estimates when compared to systems that realize independent payload estimates, determine payload attitudes. In addition, it has been found that estimating relative slave attitudes provides significant processing advantages (e.g., simpler algorithms, reduced data throughput and slower processing rate) when compared to other methods.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.